What the payoff diagram looks like. Below the put strike, the position's value is fixed: the holder sells at the strike, so further declines are somebody else's problem. Between the two strikes, the position tracks the stock exactly, because neither option is worth exercising. Above the call strike, the value is fixed again: the shares are called away at the strike, so further gains accrue to the option buyer. The collar converts an open-ended holding into a range, and the width of that range is the only real design decision.
Zero cost is a description of the cash flow, not of the economics. A so-called zero-cost collar chooses a call strike near enough to the current price that the premium received roughly equals the premium paid for the put, so nothing leaves the account on day one. The cost has not vanished; it has been paid in upside. Widening the floor, by buying a put closer to the current price, raises the put's premium and forces the call strike down to pay for it, which tightens the cap. Every collar is that same trade at a different point: more protection costs more upside, and there is no setting that provides both.
Where it is genuinely used. The classic case is an investor whose wealth sits disproportionately in one stock, often employer shares, and who cannot or does not want to sell: the gain is large and taxable, the position may be subject to trading restrictions, or a lockup has not expired. A collar buys time by removing the catastrophic downside while the holder works through the sale in an orderly way. That broader problem, and the other tools people bring to it, belong to the concentrated-position discussion rather than to this one.
The tax question, stated as the statute states it. Internal Revenue Code Section 1259 treats certain hedges of an appreciated position as a "constructive sale", which forces the holder to recognize the gain immediately as though the stock had been sold. Its list of triggers at 1259(c)(1) is specific: (A) entering a short sale of the same or substantially identical property; (B) entering an offsetting notional principal contract on it; (C) entering a futures or forward contract to deliver it; and (D) where the appreciated position is already one of those, acquiring the underlying property. None of those names an options collar. The only route by which a collar could be caught is the catch-all at 1259(c)(1)(E), which reaches "1 or more other transactions (or acquires 1 or more positions) that have substantially the same effect" as the enumerated four, and does so only "to the extent prescribed by the Secretary in regulations".
That conditional wording is the whole answer, and it is an uncomfortable one to give. A collar so tight that the floor and cap are nearly the same price leaves the holder with essentially none of the stock's remaining risk or reward, which is economically close to having sold it; a wide collar plainly does not. Where between those two the line sits is a question of guidance under paragraph (E) rather than of anything in the statute, and no percentage or strike-width safe harbor appears in Section 1259 itself. Anyone considering a collar on a substantially appreciated holding should get the analysis from a tax professional before the trade rather than after it, because the consequence of getting it wrong is an accelerated tax bill on a position the holder deliberately did not sell.
Two smaller points that are easy to get backwards. The FINRA rule quoted above ends with a condition: "Neither side of the short call/long put position can be in-the-money at the time the position is established." That is a requirement of the position-limit hedge exemption the rule is granting, not a rule about how an investor may construct a collar. And federal commodity law does use the word "collar" in a different sense: 7 U.S.C. 1a(47)(A)(i) lists "a put, call, cap, floor, collar, or similar option of any kind" among the contracts that can be a swap, where a collar is a single option-like contract on a rate, currency, commodity or other reference. The equity collar described here is three separate positions held together, not one contract.